Maintaining understory vegetation in oil palm plantations supports higher assassin bug numbers

1. The expansion of oil palm agriculture across Southeast Asia has caused significant biodiversity losses, with the reduction in habitat heterogeneity that accompanies the conversion of forest to oil palm being a major contributing factor. However, owing to their long commercial lifespan, oil palm plantations can support relatively high levels of vegetation complexity compared to annual crops. There is therefore potential for the implementation of management strategies to increase vegetation complexity and associated within-plantation habitat heterogeneity, enhancing species richness and associated ecosystem functioning within productive oil palm landscapes. 2. This study focusses on two species of assassin bugs Cosmolestes picticeps and Sycanus dichotomus, which are important agents of pest control within oil palm systems. Using a Before-After Control-Impact experimental manipulation in Sumatra, Indonesia, we tested the effect of three alternative herbicide spraying regimes and associated vegetation complexity treatments on assassin bug numbers. Our treatments encompass a range of current understory vegetation management practices used in oil palm plantations and include removing vegetation only in areas key to harvesting (“Normal”), removing all understory vegetation (“Reduced”), and allowing native vegetation to regrow naturally (“Enhanced”). We assessed both the long-term (18 months) and short-term (within 2 weeks) effects of our treatments following herbicide spraying. 3. Pre-treatment, we found high numbers of assassin bugs of both species in all plots. Long-term post-treatment, the abundance of both C. picticeps and S. dichotomus declined in Reduced understory plots, although this decline was only significant for C. picticeps (98%). In contrast, there were no significant differences in the post-treatment abundance of either species in the short-term. 4. These results suggest that the long-term decline in assassin bug abundance was likely to be caused by loss of vegetation, rather than any immediate effects of the herbicide spraying. Our findings have clear management implications as they demonstrate that maintaining vegetation in oil palm understories can benefit an important pest control agent